1. Field of the Invention
This invention relates to improvements in a four-wheel drive control system for a vehicle (automotive vehicle), and more particularly to such a system arranged to control driving forces to be distributed to front and rear wheels under control of the engaging force of a friction clutch mechanism in accordance with a rotational speed difference developed between the front and rear wheels of the vehicle.
2. Description of the Prior Art
Hitherto a variety of four-wheel drive control systems for an automotive vehicle have been proposed and put into practical use. A typical one is arranged as follows: One of front and rear (road) wheels serve as main driving wheels and others as auxiliary driving wheels. During a normal operating condition of the vehicle, whole engine power output or the greater part of the engine power output is transmitted as a "driving force" to the main driving wheels; and the driving force is distributed also to the auxiliary driving wheel under a condition where the driving force to the main driving wheel is excessive. The "driving force" transmitted to the wheels is exactly a driving torque and therefore different from a driving force which kicks a road surface and moves the vehicle; however, forces (including the driving torque) for moving the vehicle are generally referred hereinafter to as the driving force.
In this four-wheel drive control system, a friction clutch is interposed between a main propeller shaft of the main driving wheels and an auxiliary propeller shaft of the auxiliary drive wheels. More specifically, the friction clutch is interposed between the output shaft of a transmission and the auxiliary propeller shaft. A condition where the driving force to the main driving wheel is excessive is detected in accordance with the rotational speed difference between the main and auxiliary driving wheels, upon which it is recognized that the driving force to the main driving wheels is more excessive as this rotational speed difference is larger. Accordingly, the ratio of the driving force distributed amounts to the main and auxiliary driving wheels is set so that the driving force to the auxiliary driving wheel increases as the rotational speed difference increases. This four-wheel drive control system is provided with the friction clutch interposed between the transmission output shaft and the auxiliary propeller shaft, and therefore the engaging force of the friction clutch is increased as the driving force distributed amount to the auxiliary driving wheel increases.
In the vehicle provided with the above conventional four-wheel drive control system, a so-called driving force distribution feedforward control has been proposed. This control is referred hereinafter to as a "first conventional technique" and arranged as follows: A condition where slippage of the main driving wheel tends to occur has been previously detected in accordance with a depression amount (referred hereinafter to also as a "throttle opening degree") of an accelerator pedal or the like. Upon this, the ratio of the driving force distributed amounts to the main and auxiliary driving wheels is set to increase the driving force to be transmitted to the auxiliary driving wheels as the throttle opening degree increases at the starting of the vehicle, before the occurrence or increase of the slippage of the main driving wheels.
However, with the above first conventional technique, when the vehicle makes a turning-running (the vehicle runs while turning) immediately after its starting, the engaging force of the friction clutch is too high, and therefore it is impossible to absorb the rotational speed difference developed between the front and rear wheels, so that a tight corner brake phenomena (a brake is applied to the vehicle under the action of an interlock due to the rotational speed difference between the front-and rear wheels) tends to occur.
In view of this, a technique (referred hereinafter to as a "second conventional technique") disclosed in Japanese Patent Provisional Publication No. 56-138020 is known as the technique to prevent the tight corner brake phenomena from occurring even if the vehicle makes the turning immediately after the starting of the vehicle. According to this second conventional technique, the driving force (distributed amount) to the auxiliary driving wheels is set at zero when the rotational speed difference between the front and rear wheels increases over a predetermined level, so that the vehicle is put into a two-wheel driving condition.
However, the above-mentioned condition where the rotational speed difference between the front and rear wheels increases over the predetermined level can be established not only in the case that the front wheel rotational speed is over the rear wheel rotational speed when the tight corner brake phenomena occurs, but also in a case that the rotational speed of the main driving wheels is over that of the auxiliary driving wheels when the vehicle makes the starting on a low .mu. road surface. Thus, the second conventional technique makes a control upon judging only the magnitude of the rotational speed difference between the front and rear wheels. As a result, the vehicle tends to take the two-wheel driving condition during the starting on the low .mu. road surface, and therefore the main driving wheels tend to be put into a slippage condition, thereby providing problems in starting characteristics of the vehicle.